单项选择题
A. The tramp had stolen nothing of value. B. The store had profited by the incident. C. The tramp had a happy Christmas. D. The store was responsible for what happened.
A. Because people may begin a long lasting friendship or business association. B. Because people may have a nice chat. C. Because people may exchange information. D. All above.
A. The viruses can infect different species. B. The genes of the viruses come from pigs. C. The viruses mix genes from different species. D. The genes of the viruses were made artificially.